Most of the piston systems are short stroke like the HK416. In a nutshell the draw of a piston setup is the ability to keep the bolt running much cooler and cleaner since the gas never enters the upper receiver and is vented out. Supposedly this allows "over the beach" and runs much better with a suppressor on SBRs.
I like the direct impingement system and I'm not sold on the pistons being more reliable. IF and that is a big IF, I was to buy a piston AR15, it would be the PWS long stroke setup. Now don't worry about that tacticooler as reality is a hard thing for those people to deal with.